1. Field of the Invention
The present invention relates to an LC filter circuit and a laminated type LC filter.
2. Description of the Related Art
Among such LC filter circuits, for example, there is a bandpass filter having a circuit that is equivalent to that shown in FIG. 4. That is, an LC resonance circuit Q1, in which an inductor L1 and a capacitor C1 are connected in parallel, is connected between an input terminal P1 and a ground terminal. In the same manner, an LC resonance circuit Q2, in which an inductor L2 and a capacitor C2 are connected in parallel, is connected between an output terminal P2 and a grounding terminal. Then, between the inductor L1 and the inductor L2, a mutual inductance M is generated, and the resonance circuits Q1 and Q2 are magnetically coupled.
Furthermore, a series resonance trap circuit, in which an inductor L3 and a capacitor C3 are connected in series, is connected between the input terminal P1 and a grounding terminal, and a series resonance trap circuit, in which an inductor L4 and a capacitor C4 are connected in series, is connected between the output terminal P2 and a grounding terminal. Moreover, a coupling capacitor C5 is connected between the input and output terminals P1 and P2.
As described above, in the conventional LC filter circuit, the series resonance trap circuit is connected to each of the input and output terminals so as to be in parallel to the LC resonance circuit, and in such a series resonance trap circuit, a problem occurs in that the attenuation value is low and also that the frequency band is narrow.
In order to overcome the problems described above, preferred embodiments of the present invention provide an LC filter circuit and a laminated type LC filter in which a sufficient attenuation value and frequency band are achieved.
In a preferred embodiment of the LC filter circuit of the present invention, at least two LC resonance circuits, each including a parallel combination of an inductor and a capacitor and both magnetically coupled, are connected between an input terminal and an output terminal, and a parallel resonance trap circuit including a parallel combination of an inductor and a capacitor is connected in series between at least either the input terminal, the output terminal and the LC resonance circuit.
An attenuation value and frequency band required for a desired frequency can be achieved by connecting the parallel resonance trap circuit including a parallel combination of an insulator and a capacitor.
Furthermore, in the LC filter circuit of a preferred embodiment of the present invention, the parallel resonance trap circuits are preferably connected in series between the input terminal and the LC resonance circuit and between the output terminal and the LC resonance circuit, respectively. A higher attenuation value is achieved by providing the parallel resonance trap circuit on both sides.
Next, in the laminated type LC filter including therein the LC filter circuit according to the above-described preferred embodiment of the present invention, at least two LC resonators, each including a parallel combination of an inductor electrode and a capacitor electrode, one end of which is connected to an input-output terminal and the other end of which is connected to a grounding terminal, both being magnetically coupled, are provided inside a laminated body in which dielectric layers are stacked on top of one another, and a parallel resonance trap circuit including a parallel combination of an inductor electrode and a capacitor electrode, connected in series between at least one of the LC resonators and an input-output terminal, is provided inside the laminated body.
The laminated type LC filter is miniaturized by constructing the above-described LC filter circuit as a integral, unitary chip component.
Furthermore, in the laminated type LC filter of preferred embodiments of the present invention, the parallel resonance trap circuits are preferably connected in series between the input-output terminals and the LC resonators, respectively. When constructed in this way, a higher attenuation value is achieved.
Furthermore, in the laminated type LC filter of preferred embodiments of the present invention, when the parallel resonance trap circuits are connected, a shield electrode is preferably provided between the inductor electrodes of the parallel resonance trap circuits.
When constructed as described above, since the magnetic field generated by each inductor electrode is shielded by a shield electrode, the interference between the magnetic field of the inductor electrodes is minimized and adverse effects to the transmission characteristic are prevented.
Furthermore, in a laminated type LC filter of preferred embodiments of the present invention, a shield electrode is provided between the inductor electrode of the LC resonator and the inductor electrode of the parallel resonance trap circuit.
When constructed as described above, since the magnetic field generated by each inductor electrode is shielded by a shield electrode, the interference between the magnetic field of the inductor electrodes is minimized and the laminated type LC filter effectively functions as a trap circuit.
Other features, elements, advantages and characteristics of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.